1. Field of the Invention
The present invention relates to an ink jet printing apparatus and an ink jet printing method that print images using a print head capable of ejecting ink from a plurality of nozzle lines.
2. Description of the Related Art
In printing apparatus, particularly those using an ink jet print head capable of ejecting ink (ink jet printing apparatus), improvements in a printing speed during color image printing and in a printed image quality have become an important subject.
In a so-called serial scan type printing apparatus, commonly used methods for improving the printing speed include increasing a drive frequency of the print head (ink ejection frequency) and adopting a bidirectional printing system. The bidirectional printing system performs a printing scan while the print head is moving in both a forward and a backward direction. In the serial scan type printing apparatus, images are formed on a print medium successively by repetitively executing a printing scan of the print head in a main scan direction and a print medium transport operation in a subscan direction. The bidirectional printing system as a total system has a cost advantage over a one-way printing system that executes the printing scan as the print head moves only in one of the forward and backward directions because the bidirectional printing system can distribute an energy required to get the same throughput over a period of time.
In the bidirectional printing system, however, when a color image is formed by ejecting a plurality of color inks from the print heads, an order of ejecting color inks onto a print medium during the forward movement of the print head differs from that during the backward movement of the print head, giving rise to a possibility that bands of color variations may show on a printed image. Since such color variations are caused by different color ink ejection orders between the forward and backward scans of the print heads, even a slight overlapping of different color ink dots that may occur on a print medium can result in color variations to some degree.
To prevent color variations caused by the color ink ejection order difference, Japanese Patent Laid-Open No. S58-179653 (1983) discloses a print head provided with forward scan nozzles and backward scan nozzles for the same color ink. These two groups of nozzles are selectively used according to the direction of movement of the print head so that the color ink ejection order remains the same in whatever direction the print head is moving. The print head is constructed to eject, for example, Y (yellow), M (magenta), C (cyan) and Bk (black) inks.
When an ink droplet is ejected from a nozzle in response to a print signal, very fine ink droplets may also be ejected trailing a main ink droplet. Also when a main ink droplet lands on a print medium, it may bounce back from the print medium, giving rise to a possibility of very minute ink droplets being formed in a space between the print head and the print medium. Such minute ink droplets (referred to also as “ink mist”), when formed, may adhere to an ejection face of the print head (the surface of the print head formed with ejection openings), forming drops of ink on the print head. These ink drops may make the ejection of ink droplets from ejection openings unstable or cause ink ejection failures.
One method for minimizing the formation of such ink drops is by applying a water-repellent finishing to the ejection face of the print head to form a water-repellent film over the entire ejection face. In a print head with the water-repellent film, the amount of ink accumulating around the ejection openings decreases. However, where two or more nozzle lines ejecting different color inks are driven simultaneously to operate the print head at high drive frequency continuously for a long period and at high speed to form an image with high print duty, the amount of ink mist produced increases. As a result, ink drops may gradually accumulate on the ejection face of the print head.
The relation between ink mist adhering to the ejection face of the print head and an ink ejection state will be explained in the following.
As described in Japanese Patent Laid-Open No. S58-179653 (1983), color variations that may occur during a bidirectional printing can be minimized by selectively using the forward scan nozzle line and the backward scan nozzle line so that the color ink ejection order remains unchanged in whichever of the forward and backward direction the print head executes the printing scan. In the print head the forward scan nozzle line and the backward scan nozzle line are arranged symmetrically for each of different color inks.
FIG. 19 shows how an image of secondary color is formed on a print medium P by ejecting different ink droplets 11, 12 from ejection openings N1, N2 of two adjoining nozzle lines L1, L2 during the forward and backward printing scans moving in the directions of arrow X1, X2. During the forward printing scan, two of a plurality of nozzle lines forming the forward scan nozzles are used; and during the backward printing scan two of a plurality of nozzle lines forming the backward scan nozzles are used. In this example, the nozzle lines L1, L2 are formed in different chips. As the ink droplets I1, I2 are ejected from the ejection openings N1, N2 at high frequency and fly through air, nearby viscous air is pulled by the ink droplets, with the result that the proximity of the ejection face of the print head tends to be depressurized compared with the proximity of the print medium P. This causes surrounding air to flow toward the depressurized region as indicated by arrows in the figure. This air flow has been found to draw minute ink droplets, smaller than the ink droplets I1, I2 (main droplets), toward the print head. These minute ink droplets include satellites (not shown) accompanying the ink droplets I1, I2 as they are ejected and mist formed by the ink droplets I1, I2 bouncing back when they land on the print medium P.
FIGS. 20A, 20B, 21A, 21B, 22A and 22B show in such a phenomenon how mist adheres to the ejection face of the print head when a high-duty image is formed by a plurality of printing scans with a large volume of ink applied to a unit print area. In these figures, C1 and C2 denote nozzle lines for a cyan ink, M1 and M2 nozzle lines for a magenta ink, and Y1 and Y2 nozzle lines for yellow ink. Distances L between adjoining nozzle lines are 1 mm.
FIGS. 20A and 20B shows states of mist 12 formed on the ejection face of the print head when the nozzle lines C1, M2 are driven during a forward scan and the nozzle lines C2, M1 are driven during a backward scan to print a secondary color. A distance L between the simultaneously driven nozzles is 4 mm. FIGS. 21A and 21B shows states of mist 12 formed on the ejection face when the nozzle lines C1, Y1 are driven during a forward scan and the nozzle lines C2, Y2 are driven during a backward scan to print a secondary color. A distance L between the simultaneously driven nozzles is 2 mm. FIGS. 22A and 22B shows states of mist 12 formed on the ejection face when the nozzle lines C1, M1 are driven during a forward scan and the nozzle lines C2, M2 are driven during a backward scan to print a secondary color. A distance L between the simultaneously driven nozzles is 1 mm.